This essay is partly based, by permission, on an essay by Theodore S. Feldman (PSDI, Bedford, Mass), "Solar Variability and Climate Change," rewritten and expanded by Spencer Weart. For additional material, see Feldman's site.

Since it is the Sun's energy that drives the weather system, scientists naturally wondered whether they might connect climate changes with solar variations. Yet the Sun seemed to be stable over the timescale of human lifetimes. Attempts to discover cyclic variations in weather and connect them with the 11-year sunspot cycle, or other possible solar cycles ranging up to a few centuries long, gave results that were ambiguous at best. These attempts got a well-deserved bad reputation. Jack Eddy overcame this with a 1976 study that demonstrated that irregular variations in solar surface activity, a few centuries long, were connected with major climate shifts. The mechanism remained uncertain, but plausible candidates emerged. The next crucial question was whether a rise in the Sun's activity could explain the global warming seen in the 20th century? By the 1990s, there was a tentative answer: minor solar variations could indeed have been partly responsible for some past fluctuations... but future warming from the rise in greenhouse gases would far outweigh any solar effects.

When most of us think about Ice Ages, we imagine a slow transition into a colder climate on long time scales. Indeed, studies of the past million years indicate a repeatable cycle of Earth’s climate going from warm periods (“interglacial”, as we are experiencing now) to glacial conditions.

The period of these shifts are related to changes in the tilt of Earth’s rotational axis (41,000 years), changes in the orientation of Earth’s elliptical orbit around the sun, called the “precession of the equinoxes” (23,000 years), and to changes in the shape (more round or less round) of the elliptical orbit (100,000 years). The theory that orbital shifts caused the waxing and waning of ice ages was first pointed out by James Croll in the 19th Century and developed more fully by Milutin Milankovitch in 1938.

Ice age conditions generally occur when all of the above conspire to create a minimum of summer sunlight on the arctic regions of the earth, although the Ice Age cycle is global in nature and occurs in phase in both hemispheres. It profoundly affects distribution of ice over lands and ocean, atmospheric temperatures and circulation, and ocean temperatures and circulation at the surface and at great depth.

Evidence has mounted that global warming began in the last century and that humans may be in part responsible. Both the Intergovernmental Panel on Climate Change (IPCC) and the US National Academy of Sciences concur. Computer models are being used to predict climate change under different scenarios of greenhouse forcing and the Kyoto Protocol advocates active measures to reduce CO² emissions which contribute to warming.

Evidence for abrupt climate change is readily apparent in ice cores taken from Greenland and Antarctica. One sees clear indications of long-term changes discussed above, with CO² and proxy temperature changes associated with the last ice age and its transition into our present interglacial period of warmth. But, in addition, there is a strong chaotic variation of properties with a quasi-period of around 1500 years. We say chaotic because these millennial shifts look like anything but regular oscillations. Rather, they look like rapid, decade-long transitions between cold and warm climates followed by long interludes in one of the two states.

So you might say that both "global warming" and "global freezing" are inevitable. We may have made the interglacial period a little warmer this time around but, generally, there's no stopping the warming and cooling of our planet because it is stuck on the 26,000 year long carousel ride of precession.

So you might say that both "global warming" and "global freezing" are inevitable. We may have made the interglacial period a little warmer this time around but, generally, there's no stopping the warming and cooling of our planet because it is stuck on the 26,000 year long carousel ride of precession.

The so-called ‘‘little ice age’’ of the 16th–18th century may be the most recent cold phase of this cycle. The origin of the ‘‘mystery 1,500 year cycle’’ is thus one of the
key issues in climatology that needs to be explained

It should be noted that these D-O events (tied to 1470 years quasi-periodicity) and Heinrich events have not occurred during the Holocene interglacial. They appear to be a feature of glacial periods rather than interglacials. The yet to be determined "trigger" may still occur, especially if of orbital origin.

I have a hunch that the 1,470 year cycle is a permanent feature of the inner core acting dynamically against the fluid outer core. It could be thought of as the internal orbit of the inner core due to irregularities in formation.

I have a hunch that the 1,470 year cycle is a permanent feature of the inner core acting dynamically against the fluid outer core. It could be thought of as the internal orbit of the inner core due to irregularities in formation.

I have a hunch that the 1,470 year cycle is a permanent feature of the inner core acting dynamically against the fluid outer core. It could be thought of as the internal orbit of the inner core due to irregularities in formation.

We have entered a new climate epoch known as the http://en.wikipedia.org/wiki/Anthropocene" [Broken]

There is very little chance that the Earth will enter a new glacial epoch as long as humans are around. Even with extreme cutbacks in GHG emissions human activity will still provide enough positive loading of the carbon cycle to maintain a temperate to tropical Earth for at least the next million years.

As long as we are manufacturing refrigerants that get released into the atmosphere the enhanced greenhouse effect will overwhelm orbital forcings.

Humans will not allow another ice age to occur if they can help it, and they can. If the Yellowstone caldera blows there may no longer be a human civilization. So perhaps I should have said as long as human civilization exists the probability of another glaciation occuring is very remote.

At present rate of consumption, there is only about 150 years of fossil fuel reserves. While CO2 levels are currently going up around 2 ppm/year, that is not sustainable forever.

CO2 levels are currently 386 ppm. Add another 300 ppm (2*150) and the maximum is around 686 ppm. After that, levels should start to decline. We could burn everything that grows, but since plants remove CO2 from the atmosphere, that in itself won't raise CO2 levels. Of course, there is also production of cement, but the economics aren't as lucrative.

Without fossil fuels, I just don't see how we can economically keep CO2 levels elevated. In a few hundred years, levels will stabilize and then start falling. Not sure how fast, but within a few thousand years, CO2 could be below 200 ppm and with the perihelion still in January, that's a recipe for growing continental ice sheets. Maybe not a full scale ice age, but enough to be a concern.

Without fossil fuels, I just don't see how we can economically keep CO2 levels elevated. In a few hundred years, levels will stabilize and then start falling. Not sure how fast, but within a few thousand years, CO2 could be below 200 ppm and with the perihelion still in January, that's a recipe for growing continental ice sheets. Maybe not a full scale ice age, but enough to be a concern.